Heater only works on high

Sounds like 2 different problems, certainly 2 different circuits. I'd test the blower resistor which is usually the cause for the blower to only work on high. The mode door actuator is the component that handles moving your air distribution from defrost to vent to floor etc. could be the switch, could be the actuator, could be a nest keeping door from moving, but you'd start with testing the inputs to the actuator and seeing if it even works.